1. Field of the Invention
The present invention relates to a current detector circuit incorporated in a power semiconductor device, for detecting a main current flowing through an output main element and, more particularly, a current detector circuit for detecting a current flowing through a sense element for sensing part of the main current.
2. Description of the Related Art
In a power semiconductor integrated circuit device (e.g., a power integrated circuit device) having a power switching element such as an insulated gate bipolar transistor (IGBT) and a metal oxide semiconductor field effect transistor (MOSFET) for switching a large load current, a current detector circuit is often incorporated to detect an overcurrent of the power switching element. A protection circuit is also incorporated to protect the integrated circuit device from thermal breakdown caused by the overcurrent.
FIG. 8 and FIG. 9A show conventional current detector circuits, respectively.
To detect a main current Io flowing through an IGBT 1 serving as an output main element, the current detector circuit of the first prior art shown in FIG. 8 causes an IGBT 2 serving as a sense element to sense part of the main current. When the sense current Is flows through a current detection resistor element Rs, a voltage drop Vs occurs across the resistor element Rs. When the voltage drop Vs exceeds the base-emitter threshold voltage of a control transistor 3 (npn transistor in this example), the control transistor 3 is triggered to flow a control current Icont from a bias power supply circuit through the control transistor 3. When the control current Icont flows, a control signal corresponding to the control current Icont is output from a current amplifier circuit 25 functioning as a control circuit. The overcurrent of the main element is detected using this control signal, thereby causing a protection circuit (not shown) to protect the main element from the overcurrent.
In the circuit of the first prior art, however, the base-emitter threshold voltage of the control transistor undesirably changes depending on a temperature dependence in accordance with a temperature rise (change in temperature) caused by currents flowing through the respective parts during an operation.
The current detector circuit as the second prior art shown in FIG. 9A is a circuit improved for compensating for the temperature dependence of the base-emitter threshold voltage of the control transistor 3 in the current detector circuit of the first prior art. The circuit of the second prior art is different from the circuit of the first prior art in that a diode 12 having substantially the same temperature dependence as that of the base-emitter threshold of the control transistor 3 is inserted in series with the current detection resistor element Rs. The diode 12 is formed by a p-n junction which has substantially the same characteristic as that of the base-emitter p-n junction of the control transistor 3.
The operation of the circuit of the second prior art is basically almost the same as that of the circuit of the first prior art, except for an operation associated with temperature compensation by the diode 12. When the base-emitter threshold voltage of the control transistor 3 changes depending on its temperature dependence, a forward voltage drop of the diode 12 inserted in series with the current detector resistor element Rs (detection resistor) changes depending on its temperature dependence. Since the temperature dependence of the control transistor 3 is almost equal to that of the diode 12, a change in base-emitter threshold voltage of the control transistor 3 can be compensated for.
In each prior art described above, the sense element 2 may be monolithically formed in an integrated circuit chip having the main element 1 on a silicon substrate. Alternatively, the sense element 2 may be formed in an integrated circuit chip different from that having the main element, and the resultant sense element integrated circuit chip may be externally connected to the main element integrated circuit.
In the circuit of each prior art described above, current detection is performed under an assumption that the main current Io and the sense current Is have a predetermined ratio (e.g., n:1). That is, the following equation is established: EQU Io=n.Is
Note Vs can be expressed by the following equation: EQU Vs=Rs.Is
so that the main current Io can be expressed using the above two equations: EQU Io=n.Vs/Rs (1)
The control current Icont flowing through the control transistor 3 and the main current Io flowing through the main element IGBT 1 are assumed to be operated to satisfy a predetermined relationship. When a preset value is determined for the control current Icont in advance based on the assumption, the value of the main current Io can be controlled.
However, in an actual operation, the relation represented by equation (1) cannot be necessarily established. A protection operation for the main element on the basis of detection of a predetermined level of the main current Io, i.e., a normal protection operation may not be realized. This phenomenon may be estimated due to the following reasons (A) and (B).
(A) In the circuit of the first prior art, the above phenomenon is caused by an increase in temperature dependence of a base-emitter voltage VBE (or a gate-source voltage VGS in a MOS transistor) of the control transistor 3 (when a temperature increases, the voltages VBE and VGS are generally lowered). More specifically, as shown in FIG. 11, a threshold voltage Vth of the voltage VBE (or VGS) of the control transistor 3 is greatly changed with temperature changes. For this reason, the detection level for an overcurrent of the main current Io corresponding to the voltage drop Vs (=Rs.Is) across the detection resistor Rs so as to turn on the control transistor 3 is greatly changed with temperature changes, as shown in FIG. 12 (the detection level of the overcurrent of the main current Io normally is lowered at higher temperatures). Therefore, the protection operation against the overcurrent cannot be affected.
(B) Although the circuit of the second prior art is an improved circuit of the first prior art so as to compensate for the temperature dependence of the base-emitter threshold voltage of the control transistor 3, the above phenomenon occurs because a predetermined sense ratio (=Io/Is) cannot be obtained.
That is, the equivalent circuit of the circuit of the second prior art is shown in FIG. 9B, and the following equation is established: EQU Vs=Io.Rs.(VCESAT-VF)/(n.VCESAT+Io.Rs)
where VCESAT is the saturation voltage of the circuit including the main element circuit and the sense current circuit, when viewed across the external terminals C and E.
At this time, when a forward voltage drop VF of the temperature compensation diode is sufficiently smaller than the voltage value VCESAT, and the voltage drop Vs across the detection resistor Rs is sufficiently lower than VCESAT, that is, when the following conditions are satisfied:
VCESAT&gt;&gt;VF
VCESAT&gt;&gt;Rs.Is=Rs-Io/n equation (2) is rewritten as follows:
Vs=Rs.Io/n which is equal to equation (1).
In practice, however, VF=0.5 to 0.6 V is obtained for VCESAT=2 to 3 V, which does not satisfy the conditions for establishing equation (1).
In an actual operation, the trigger level of the control transistor 3 cannot be substantially constant with respect to the main current Io. The problem in which the protection operation against the overcurrent cannot be stably effected like the circuit of the first prior art is left unsolved.
In addition, as can be apparent from equation (1), Rs must also be set sufficiently low, and the base-emitter threshold voltage of the control transistor 3 and the sense current value must be carefully designed.
As shown in the characteristics (actual measurement data representing linearity of detection current vs. output current in FIG. 13, each of the circuits of the first and second prior arts is found to have a large current detection offset (i.e., an operation delay range until a detection current flows upon flowing the main current Io). This large current detection offset may be caused because the voltage drop Vs across the detection resistor Rs or the voltage Vs+VF cannot be neglected under nominal operation range of the main current Io (i.e., a range in which VCESAT is low), and the detection current Is of the current detection IGBT 2 tends not to flow. Therefore, the circuits of the first and second prior arts cannot accurately detect currents.
In the current detector circuit of the first prior art has a temperature dependence on the base-emitter threshold voltage of the control transistor 3. The overcurrent detection level of the main current corresponding to the voltage drop across the detection resistor at the turn-on occasion of the control transistor 3 is greatly varied depending on temperatures. The protective operation against the overcurrent cannot be normally performed on the basis of the detection current output.
Although the current detector circuit of the second prior art can compensate for the temperature dependence on the base-emitter threshold voltage of the control transistor 3, a constant sense ratio n (=Io/Is) of the main current Io to the sense current Is cannot be obtained. As a result, the protection operation against the overcurrent cannot be desirably performed.